Age Readings of Sebastes marinus and S. mentella Otoliths: Bias and Precision Between Readers

Abstract

This study presents comparative otolith-based age readings of golden redfish (Sebastes marinus) from Iceland and demersal as well as pelagic deep-sea redfish (Sebastes mentella) from East Greenland and the Irminger Sea within an otolith exchange program between institutes in Germany, Iceland, Norway and Spain. A total of 571 otoliths were thin-sectioned and read independently in the participating labs. Age reading results were compared between readers in terms of bias and precision, using a set of statistical tests and graphical methods. Significant bias was observed between readers, mainly caused by deviations between age scores in the higher ages (> 20 years). Percent agreement was poor (< 30%) for a tolerance level of ± 0 years, particularly for the age range 21-40 years, which represents the major fraction of the fished stock. A tolerance level of ± 5 years, however, lead to up to 95% agreement for the age ranges up to 20 years. Precision and bias were generally better for S. marinus than they were for S. mentella, pointing to greater difficulties in the interpretation of growth stuctures for the latter species. The mean age of S. mentella with total lengths of 25-30 cm, which were observed to have recruited from the East Greenland shelf into the Irminger Sea during 1998/99, was determined to be 9-10 years. The observed problems in bias and precision of age readings should to be improved by continuing with similar Sebastes otolith exchange programs and setting up a further age reading workshop to harmonise the interpretation of growth structures. Radiometric ageing is currently being finalised and results will be available for the next NWWG meeting.